The manufacture of micro-machined electromechanical sensor (MEMS) force and acceleration measurement devices is generally well-known for many different applications. Some applications require the device to provide very accurate measurements, while other applications require the device to withstand extreme shock and vibration environments.
Some applications require the measurement of force in extreme dynamic environments. For example, if a gun-launched projectile requires on-board acceleration sensing, the accelerometer providing the sensing must have a high pickoff sensitivity, and must be capable of operating in a high-g range with high-g shock survivability characteristics for shock loads in the range of 16,000 to 20,000 g's or more. Capacitive-sensing MEMS accelerometer devices are often used for operation in such high-g environments because they are capable of a small overall geometry that is ideal for high-g sensor packages that operate in a minimum space.
Other applications require the measurement of minimal forces in the micro-g range. These applications may be even more sensitive to even minimal shock and vibration environments as the suspension structure supporting the reaction mass must be responsive to these very minimal input loads.
The emphasis on high pickoff sensitivity in minimum space encourages the use comb-type capacitive pickoff sensors having large quantities of pickoff electrodes arranged on both a moveable proof mass and a fixed base with the opposing pickoff electrodes overlapping like the teeth of two combs. Maximizing pickoff sensitivity requires maximizing the capacitance between the overlapping electrodes. Capacitance may be maximized by increasing travel of the proof mass. But this must be accomplished without overstressing the proof mass suspension. Typically, shock stops are strategically placed to limit the travel of the proof mass so as to protect the proof mass suspension from overstress. Limiting the travel of the proof mass also protects the overlapping electrodes from coming into harmful contact.
Capacitance also may be maximized by increasing the number of interacting sensing electrodes. However, the drive toward smaller package sizes forces a tradeoff between increasing the number of sensing electrodes and effectively locating sufficient shock stops to effectively limit the travel of the proof mass.
Additionally, an accelerometer in a gun-launch application must also exhibit low cross-axis sensitivity characteristics. Today's designers are challenged when faced with a need to contain all of these features in a low cost, small size accelerometer device.
One solution is described in published U.S. patent application Ser. No. 10/117,303, “SMALL SIZE, HIGH CAPACITANCE READOUT SILICON BASED MEMS ACCELEROMETER,” filed Apr. 5, 2002, in the name of Ronald B. Leonardson, now U.S. Pat. No. 6,705,166, the complete disclosure of which is incorporated herein by reference, which uses overlapping concentric continuous annular electrodes on opposing base and proof mass plates with the proof mass suspended by an annular flexure.
Another attempted solution is described by Biebl, et al. in U.S. Pat. No. 5,447,067, “ACCELERATION SENSOR AND METHOD FOR MANUFACTURING SAME,” the complete disclosure of which is incorporated herein by reference, which relies on elongated proof mass suspension flexures to provide extended proof mass travel without damage to the suspension. Unfortunately, the Biebl, et al. solution ignores the need to fit the sensor in a small package.